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Electromagnetic (EM) tracking systems are often used for real time navigation of medical tools in an Image
Guided Therapy (IGT) system. They are specifically advantageous when the medical device requires tracking
within the body of a patient where line of sight constraints prevent the use of conventional optical tracking. EM
tracking systems are however very sensitive to electromagnetic field distortions. These distortions, arising from
changes in the electromagnetic environment due to the presence of conductive ferromagnetic surgical tools or
other medical equipment, limit the accuracy of EM tracking, in some cases potentially rendering tracking data
unusable. We present a mapping method for the operating region over which EM tracking sensors are used,
allowing for characterization of measurement errors, in turn providing physicians with visual feedback about
measurement confidence or reliability of localization estimates.
In this instance, we employ a calibration phantom to assess distortion within the operating field of the
EM tracker and to display in real time the distribution of measurement errors, as well as the location and
extent of the field associated with minimal spatial distortion. The accuracy is assessed relative to successive
measurements. Error is computed for a reference point and consecutive measurement errors are displayed relative
to the reference in order to characterize the accuracy in near-real-time. In an initial set-up phase, the phantom
geometry is calibrated by registering the data from a multitude of EM sensors in a non-ferromagnetic ("clean")
EM environment. The registration results in the locations of sensors with respect to each other and defines
the geometry of the sensors in the phantom. In a measurement phase, the position and orientation data from
all sensors are compared with the known geometry of the sensor spacing, and localization errors (displacement
and orientation) are computed. Based on error thresholds provided by the operator, the spatial distribution of
localization errors are clustered and dynamically displayed as separate confidence zones within the operating
region of the EM tracker space.
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